Coagulation is a physiological pathway involved in maintaining normal blood hemostasis in mammals. Under conditions in which a vascular injury occurs, the coagulation pathway is stimulated to form a blood clot to prevent the loss of blood. Immediately after the vascular injury occurs, blood platelets begin to aggregate at the site of injury forming a physical plug to stop the leakage. In addition, the injured vessel undergoes vasoconstriction to reduce the blood flow to the area and fibrin begins to aggregate forming an insoluble network or clot, which covers the ruptured area.
When an imbalance in the coagulation pathway shifts towards excessive coagulation, the result is the development of thrombotic tendencies, which are often manifested as heart attacks, strokes, deep vein thrombosis, and acute coronary syndromes such as myocardial infarcts, and unstable angina. Furthermore, an embolism can break off from a thrombus and result in a pulmonary embolism or cerebral vascular embolism including stroke or transient ischemia attack. Current therapies for treating disorders associated with imbalances in the coagulation pathway involve many risks and must be carefully controlled.
Heparin and low molecular weight heparins (LMWHs), complex, sulfated polysaccharides isolated from endogenous sources, are potent modulators of hemostasis. Heparin, a highly sulfated heparin-like glycosaminoglycan (HLGAG) produced by mast cells, is a widely used clinical anticoagulant, and is one of the first biopolymeric drugs and one of the few carbohydrate drugs. Heparin and molecules derived from it are potent anticoagulants that are used in a variety of clinical situations, especially for thromboembolic disorders including the prophylaxis and treatment of deep venous thrombosis and pulmonary embolism, arterial thromboses, and acute coronary syndromes like myocardial infarction and unstable angina. Heparin and LMWHs interact with multiple components of the coagulation cascade to inhibit the clotting process. Heparin primarily elicits its effect through two mechanisms, both of which involve binding of antithrombin III (AT-III) to a specific pentasaccharide sequence, HNAc/S,6SGHNS,3S,6SI2SHNS,6S contained within the polymer. First, AT-III binding to the pentasaccharide induces a conformational change in the protein that mediates its inhibition of factor Xa. Second, thrombin (factor IIa) also binds to heparin at a site proximate to the pentasaccharide/AT-III binding site. Formation of a ternary complex between AT-III, thrombin and heparin results in inactivation of thrombin. Unlike its anti-Xa activity that requires only the AT-III pentasaccharide-binding site, heparin's anti-IIa activity is size-dependent, in addition to the pentasaccharide unit responsible for anti-Xa activity for the efficient formation of an AT-III, thrombin, and heparin ternary complex. Heparin also mediates the release of tissue factor pathway inhibitor (TFPI) from endothelial cells. TFPI, a heparin cofactor, is a serine protease that directly binds to and inhibits factor X. TFPI is a potent anti-thrombotic, particularly when co-administered with heparin.
Although heparin is highly efficacious in a variety of clinical situations and has the potential to be used in many others, the side effects associated with heparin therapy are many and varied. Anti-coagulation has been the primary clinical application for unfractionated heparin (UFH) for over 65 years. Due to its erratic intravenous pharmacokinetics and lack of subcutaneous bioavailability, UFH has been administered by intravenous injection instead. Additionally, the application of UFH as an anticoagulant has been hampered by the many side effects associated with non-specific plasma protein binding with UFH.
This has led to the explosion in the generation and utilization of low molecular weight heparin (LMWH) as an efficacious alternative to UFH. LMWH provide a more predictable pharmacological action, reduced side effects, and better bioavailability than UFH. Since the commercially available LMWH preparations are not fully neutralized by protamine, an unexpected reaction could have extremely adverse effects; the anti-Xa activity of enoxaparin and other LMWH are neutralizable only to an extent of about 40% with ≦2 mg Protamine/100 IU anti-Xa LMWH. The anti-IIa activity is neutralizable only to an extent of about 60% with ≦2 mg Protamine/100 IU anti-Xa LMWH. (On the other hand, the anti-Xa and anti-IIa activity of UFH is neutralizable almost completely (>90%) with ≦3 mg Protamine sulfate/100 IU anti-Xa UFH.)
Pharmaceutical preparations of these polysaccharides, typically isolated from porcine intestinal mucosa, are heterogeneous in length and composition. As such, only a portion of a typical preparation possesses anticoagulant activity. At best, the majority of the polysaccharide chains in a pharmaceutical preparation of heparin or LMWH are inactive, at worst, these chains interact nonspecifically with plasma proteins to elicit the side effects associated with heparin therapy. Therefore, it is important to develop novel LMWHs that retain the anticoagulant activity and other desired activities of UFH but have reduced side effects. LMWHs, essentially due to their reduced chains sizes and dispersity, display markedly less non-specific plasma protein binding. However, all LMWHs that are currently clinically available also possess reduced anti-IIa activity as compared to UFH. Because of this decreased activity, a larger dose of LMWH is required (compared to UFH) in order to achieve a similar anti-Xa and anti-IIa activity, and the standard tests for UFH activity, activated partial thromboplastin time (aPTT) or activated clotting time (ACT), are not useful as they rely primarily on anti-IIa activity for a readout. The most widely used test for monitoring LMWH levels is an anti-Xa activity test, which depends on the subject having sufficient levels of antithrombin III (ATIII), which is not always the case. This test is quite costly (well over $100.00) and is not routine or readily available, as samples generally must be sent to an outside lab for analysis. Consequently, the use of LMWHs so far has been largely limited to the prevention of thrombosis and not to their treatment, and the population of patients to whom it can be administered has been limited, excluding, among others, pediatric patients, patients with abnormal renal function as measured by RFI, urea, creatinine, phosphorus, glomerular filtration rate (GFR), or BUN (Blood Urea Nitrogen level) in blood and urine and the interventional cardiology patient population.